What would be a good price for this headphone and is it worth getting? I'm asking because I see a pretty cheap one for sale nearby. It's ad price is 140 euro. (around 180 dollars or sumthing) I can probably get some off the price. BUT it comes as is. He couldn't test it and I myself don't have an amp.

Is it possible to run these out of a speaker amp someway? Maybe with a converter, if it's possible what do I need for not too much money.

Obviously the guy got it used and/to flip or he would have known if it worked to begin with. May be completely shot or have channel imbalance.

The foam inside may be deteriorated?

No you cannot run it directly off a speaker amp without a Stax Pro Bias adaptor unit that changes the input signal voltage up to what the headphone needs= 580 volts.

These hook up to your amp's speaker terminals to get the signal from them and step it up ( SRD-7 Pro, SRD-7 MK2, WOO WEE )

Personally, without testing and confirmation that it works and also works without one side louder than the other, or any sort of driver buzz /rattling I ndefinitely would steer clear, unless you are willing to risk it.

But you still need to find a pro bias Stax Adaptor and they are not cheap, or a Stax energizer which is basically a small amp you feed the signal into.

So your total outlay costwise will be at least double what you pay for the headphones themselves. At LEAST. As squallkiercosa mentioned above also.

It is a different sound than HD 650 as it has different technology. Speed, detail, natural sounding "righr" timbre - is unparalled due to electrostatic transducers and it is a chance that after acquiring the stax you would notice that the highly regarded HD 650 has somewhat plastic sound and lacking details and especially micro-dynamism. But you need or an energizer, which connects to your power (integrated) amplifier /that is cheap but good solution) or a separate stax headphone amplifier, which is a pricier option, especially if you want a better than the energizer option.